To reduce the cost of printed wiring board manufacture, a solder mask layer has been placed directly over bare copper wire traces etched on a copper clad substrate. Connections are made to the wire traces, at circuit pad connector positions commonly called lands, by exposing the copper, that is by leaving voids in the solder mask so that the connection areas or circuit pads including those at through hole positions are not covered. The circuit pads are then covered with solder.
The copper cladding on the substrate is about 0.0014 in. (0035 cm) thick to facilitate etching of the circuit traces. Before etching the copper traces are electroplated to full thickness of about 0.002 in. (0.05 cm) before applying the solder mask. Also it is conventional to selectively plate the circuit pad positions with tin-lead or other protective layers before applying the solder mask layer and before etching. This requires a step of stripping the tin-lead layer in a chemical process which is difficult to protect against pollution.
The prior art process also limits the wiring density, since the land areas need to be in the order of 0.050 in. (0.13 cm) in diameter in order to make a reliable connection. Thus a closer together spacing of circuit traces is not feasible.
It is therefore an object of this invention to correct the foregoing deficiencies of the prior art. Further objectives are to reduce pollution in the manufacturing process, to increase wiring density, to reduce the amount of copper on the circuit boards, and to decrease the number of manufacturing steps and therefore the cost of production of printed circuit boards.